Abstract

Optical antennas have been widely used for sensitive photodetection, efficient light emission, high resolution imaging, and biochemical sensing because of their ability to capture and focus light energy beyond the diffraction limit. However, widespread application of optical antennas has been limited due to lack of appropriate methods for uniform and large area fabrication of antennas as well as difficulty in achieving an efficient design with small mode volume (gap spacing < 10nm). Here, we present a novel optical antenna design, arch-dipole antenna, with optimal radiation efficiency and small mode volume, 5 nm gap spacing, fabricated by CMOS-compatible deep-UV spacer lithography. We demonstrate strong surface-enhanced Raman spectroscopy (SERS) signal with an enhancement factor exceeding 108 from the arch-dipole antenna array, which is two orders of magnitude stronger than that from the standard dipole antenna array fabricated by e-beam lithography. Since the antenna gap spacing, the critical dimension of the antenna, can be defined by deep-UV lithography, efficient optical antenna arrays with nanometer-scale gap can be mass-produced using current CMOS technology.

Numerical simulations of gold arch-dipole antenna array. (a) Schematic of the simulated structure. 210 nm long, 50 nm wide, and 40 nm thick gold arch-dipole antennas with 5 nm gap and 30 nm arch height were simulated. Periodic boundary condition was used to calculate an antenna array with 600 nm pitch. (b) Electric field enhancement of arch-dipole antenna (red curve). Standard dipole antenna with same gap and length was also simulated for comparison (blue curve). Arch-dipole antenna has two modes depending on the current direction in the arch. (c) Electric field magnitude profile of simulated arch-dipole antennas. White arrows in antennas represent the current distribution. (d)-(f) Simulations of arch-dipole antennas with various arch heights. Field enhancement spectra show a maximum from arch-dipole antennas with 30 nm arch height (d). Quality factor (e) and field enhancement (f) were also plotted as a function of arch height. The field enhancement has a maximum peak at the optimum arch height for Q-matching condition (Qrad = Qabs).